An exhaust gas discharged in a large quantity from a blast furnace is maintained at a high temperature and a considerably high pressure. When the blast furnace exhaust gas is washed with water in a dust precipitator or the like disposed downstream, the temperature is lowered to a level approximating to ambient temperature but it still retains a sufficient pressure energy. In iron manufacturing plants, one of the important problems is how to attain the energy-saving effect how to effectively recover such pressure energy possessed by a blast furnace exhaust gas.
As a most conveniently workable method for recovery of such pressure energy, there can be mentioned a method in which a turbine is driven by utilizing the blast furnace exhaust gas and a power generator is driven by this turbine to convert the pressure energy to electric energy.
This energy recovery method, however, still involves problems to be solved. In the first place, the pressure or amount discharged of the blast furnace exhaust gas is not constant, but in general, it is readily changed depending on the resistance in the blast furnace, namely depending on the molten state of ore or the flow state of blast furnce slag. During the operation in the blast furnace, a part of the packed material is molten and coagulated and is often suspended in the blast furnace. When this suspended state becomes impossible to keep, the coagulated mass is let to fall down and a large quantity of a high temperature gas is blown out at a time. Namely, the so-called "blow-out" phenomenon is caused to occur.
In the normal operation of the blast furnace, the temperature of the gas at the outlet of the blast furnace is 200.degree. to 250.degree. C. Since this high temperature is cooled by water sprayed from a venturi scrubber or the like of a dust precipitator disposed between the outlet of the blast furnace and the turbine, the temperature of the gas at the inlet of the turbine is lowered to 60.degree. to 80.degree. C. However, when the blow-out phenomenon takes place, the temperature of the gas at the inlet of the turbine is elevated to 250.degree. C. or higher. Accordingly, because of elongation of the turbine rotor or moving blade or uneven distortion of a casing, such undesirable phenomenon as abnormal contact of the top end of the moving blade with the casing is caused to occur or the overload is imposed on the power generator.
Secondarily, dusts contained in the exhaust gas adhere to the transportation passage or the turbine, especially a stationary blade thereof, and these dusts disturb gas flows and reduce the efficiency of the turbine.
As means for preventing adhesion and accumulation of dusts to the casing inlet and stationary blade of the turbine, there is adopted a method in which parts to which dusts are likely to adhere are coated with a material having a good parting property, such as a fluorine resin, a phenolic resin or crystalline metal oxide ceramics.
When the interior of the turbine is coated with such material having a good parting property, it is possible to prevent adherence of dusts and resulting reduction of the efficiency of the turbine, but because such coating material lacks heat resistance or readily undergoes thermal degradation, if the above-mentioned "blow-out" phenomenon takes place in the blast furnace, by a high temperature gas instantaneously introduced into the transportation passage and the turbine, the coating material is thermally degraded.
In the blast furnace, packed materials such as ore change their shapes moment by moment, and therefore, it is impossible to prevent occurrence of the "blow-out" phenomenon. Accordingly, the turbine system must be designed and arranged so as to cope with this unavoidable "blow-out" phenomenon.